Bordetella pertussis produces several protein toxins including pertussis toxin and dermonecrotic toxin or heat labile (HLT). HLT, when injected subcutaneously into rabbits or sucking mice, results in a local pronounced hemorrhagic lesion. The extent, localization and nature of this toxin-induced innury suggests HLT contributes to the pathogenesis of B. pertussis. Conditions for assay of the toxin using the suckling mouse model have ben established. By use of conventional protein purification techniques and high pressure liquid chromatography, nearly homogeneous preparations of HLT have been obtained. These studies show that the toxin is a single polypeptide chain with a molecular weight about 150,000. During the course of these studies it was observed that adenylate cyclase activity copurifies with HLT during the early states of purification. However, these components can be resolved during later stages of purification. Studies with ATP analogs and other agents indicate that HLT and the adenylate cyclase exhibit similar sensitivity to inhibition. These data suggest a possible relation between adenylate cyclase and HLT, further studies are necessary. Preliminary data suggests that the action of proteolytic enzymes on purified preparations of HLT lead to an increased adenylate cyclase activity in these preparations. Thus, a proenzyme enzyme relation is possible.